Soil warming duration and magnitude affects dynamics of fine-roots and rhizomes and C and N pools in belowground biomass in subarctic grasslands

Climate predictions for arctic and subarctic regions show a higher rise in surface temperature than the global mean, which will subsequently raise the soil temperature (Ts) in those regions. We investigated the effects of soil warming duration (medium-term (11-yr) vs. long-term (>60-yr) warmed grassland) and magnitude from +0.2 to +6.2 °C on total belowground plant biomass (BPB) as well as in two functional groups: short-living fine-roots and long-living rhizomes in topsoil (0-10cm) and subsoil (10-30cm). We also analyzed the effect of plant community composition on belowground biomass and pools.

Both the duration and the magnitude of soil warming influenced the dynamic of total belowground biomass (BPB) and fine-roots and rhizomes separately. The soil warming effect varied along the soil depths. Both changes in carbon and nitrogen concentration in fine-roots and rhizomes and their corresponding biomass contributed to the significant decline in carbon and nitrogen pool in belowground plant biomass along the warming gradient. The change in the functional structure of the plant community was related to the increase in soil temperature. The proportion of forbs increased towards warmer plots and was related to the change in the BPB and soil chemistry. Our findings underline the importance of a functional approach in root research to understand better the key physiological processes like N and C cycling. We highlight the role of soil chemistry and community changes together with warming (duration and magnitude) in the fine-root and rhizome response to climate change.